1. Field of the Invention
The present invention relates generally to position sensors and, more particularly, to sensors for sensing the position of a linear motor relative to a platen.
2. Description of the Prior Art
Linear motors are well known in the robotics art. For example, a Sawyer linear motor disclosed in U.S. Pat. No. 3,376,578 can provide linear motion over two mutually orthogonal directions in the plane and a small rotation in the plane. These electromagnetic linear motors move on an air bearing over a ferromagnetic platen with a cyclic magnetic pattern in it along its two planar axes. Currently, these linear motors are controlled as step motors with no position feedback.
With no position sensing, planar Sawyer motors are operated as step motors with a micro-stepping controller that has several limitations. The motor can lose synchronization when external forces are applied or when commanded to move at high speed. When the motor loses synchronization its position is no longer known and thus the motor system is unreliable. More common is for the motor to lose rotational synchronization which cannot be detected or corrected by the motor itself. The motor rotations cause loss of motor performance and position errors. As a result of open loop motor control with respect to position, the motor has a position error because of disturbing forces especially at steady-state and low frequency when acceleration and velocity feedback is not effective. In addition, the motor cannot be electronically commutated to obtain the maximum force. Moreover, the motor requires full current even when only low force is needed. Higher currents lead to higher motor heat dissipation causing larger thermal expansion of the platen thereby producing larger position errors of the motor.
This type of linear motor is commonly referred to as a "forcer". The forcer is comprised of pairs of linear motors that are combined to provide two axes of motion (x-y) in mutually perpendicular directions. The forcer is moved across: a flat platen electromagnetically. The forcer can be actuated in the x or y direction separately or in a vectorial combined direction. However, the forcer can only be rotated about the z axis through small angles.
One and two dimensional linear stepping motor systems (forcers) generally operate in an open loop fashion. Currents are sent through the motor windings with two, three or four phases and are generally advanced-through computer control by "micro-stepping" with either linear or pulse-width modulated (PWM) drives. This permits the motor to reach positions which are multiples of very small fractions of the motor's natural period based on teeth spacing. To insure that no steps are missed, generally only two-thirds to three-fourths of the available force is used. However, this mode of operation reduces the motor's potential maximum acceleration and velocity performance, makes it susceptible to loss of synchrony (missing steps), has a longer than desirable settling time after moving, and requires high power dissipation when holding a position to obtain acceptable stiffness.
There are several known techniques that can be used to measure position in a one or two dimensional linear motor system including those that can rotate. Such techniques include laser interferometry, tracking from light sources attached to the motor, optical sensing of teeth in the platen, capacitive sensing of the teeth and magnetic sensing of the teeth. With regard to the latter, for example, U.S. Pat. No. 3,735,231 discloses an inductive sensor which can be integrated with a Sawyer motor. One embodiment of the sensor in U.S. Pat. No. 3,735,231 includes a four pole magnetic member having a pair of sense windings which can be in the form of a printed circuit board disposed on non-adjacent poles at the exposed ends of the poles. The pair of windings provide outputs which are a periodic function of the head relative to the platen along one axis.
In another embodiment of the sensor disclosed in U.S. Pat. No. 3,735,231, the sensor comprises two inverted u-shaped ferromagnetic members disposed in a parallel and spaced relationship to each other by a magnetic cross bar. A drive winding is disposed on the cross-bar and a sense winding is wrapped around each of the four poles of the sensor. When the drive winding is energized with a periodic signal the two sense coils of each inverted u-shaped member produce a periodic voltage as the sensor is moved along a single axis. The output from each inverted u-shaped member is in phase quadrature and can be used to determine the displacement of the motor head relative to the platen along a single axis. A similar arrangement can be used to indicate displacement along an axis perpendicular to the single axis.
Another magnetic sensor for 2-D linear stepping motors is disclosed in Brennemann et al., "Magnetic Sensor for 2-D Linear Stepper Motor", IBM Technical Disclosure Bulletin, Vol. 35, No. 1B (June 1992). This sensor is an ac magnetic sensor based on self inductance of coils integrated with a sawyer motor. The sensor includes four linearly arranged poles, each having a plurality of teeth. Two poles on the left are separated from the two poles on the right by a magnetic spacer. A sense coil (L.sub.1 -L.sub.4) is wound around each of the poles. The sensor for each axis consists of eight coils wound on eight poles. Four poles are positioned in one quadrant of the forcer and four are positioned in the diagonally opposite quadrant. The inductance of a first sense coil L1 is at a maximum when the inductance of an adjacent sense coil L2 is at a minimum and vice-versa. The sense coils L3 and L4 are in phase quadrature with the coils L1 and L2. Each four pole sensor produces quadrature related output voltages which vary sinusoidally with motor displacement along one axis of the platen. The sensors can be used to measure displacement along one of two axes and rotation about the z axis. However, the above two sensors are relatively complex to make and use, expensive to manufacture, difficult to shield from unwanted external fields and have a relatively small signal.
U.S. Pat. No. 3,857,078 discloses a closed loop Sawyer motor system using magnetic position sensing. For detection along each axis, two pickoff assemblies are utilized. Each pickoff includes two magnetic cores joined by a magnetic cross piece having a drive coil wrapped around it. Each magnetic core has two poles, each with three teeth. The two poles of one core are spaced in a phase quadrature relationship with the two poles of the other core. The flux in each core varies with the linear positioning of the pickoff relative to the platen and the fluxes in the two cores are in a quadrature relationship with each other. A sense coil is wound around an upper horizontal portion of each core member. The two sense coils provide quadrature related output signals having periodic relationships in accordance with the actual displacement of the head along the platen. However, this sensor suffers from the disadvantage that since the magnetic path is not symmetrical on both sides of the drive coil the outputs have a large common mode (bias field) component which is not canceled.
U.S. Pat. No. 4,737,698 discloses an inductive sensor with coupled planar drive and sense windings in conjunction with a moveable electrically conductive screen whose position is determined by the degree of coupling between sense and drive. The conductive screen varies the coupling between drive and sense windings by the principle of eddy current shielding. When the drive winding is excited a voltage induced in the sense winding provides a signal indicative of the screen position. However, the sensor does not measure displacement of a coil structure with respect to a toothed magnetic structure. Moreover, the presence of the conducting screen decreases the coupling between drive and sense windings. In addition, the sensor has air-core coils.
Zeevi and Ish-Shalom, IEEE Tran. on BME, pp. 511-522, July 1982 describes an eye movement measurement device having one coil drive and two sense coils that reduce the effects of stray fields on measurement and simplifies the measurement method, because no bridge is required.
Thus, there is a need to develop a position sensor that can sense a position of a linear motor in the plane (x, y and rotation) that provides accurate position sensing during closed and open loop operation, eliminates any common mode pick up, is simple, low cost, can be integrated with a sawyer motor and can operate with multiple motors on one platen.